The technical field of the invention is the field of manufacturing and installing bottom-to-surface production connections for offshore extraction of oil, gas, or other soluble or fusible material, or of a suspension of mineral matter, from an undersea wellhead in order to develop production fields installed offshore, at sea. The main and immediate application of the invention lies in the field of producing oil, and also in the field of reinjecting water, and in the fields of producing or reinjecting gas.
In general, a floating support includes anchor means enabling it to stay in position in spite of the effects of currents, winds, and swell. It generally also has drilling means, oil storage and processing means, and also means for off-loading onto oil-removing tankers which call at regular intervals to remove production. Such floating supports are known as “floating production storage off-loading” supports and the initials “FPSO” are used throughout the description below or else they are known as floating and drilling production units (FDPU) when the floating support is also used for performing drilling operations with a well deflected in the depth of the water.
A catenary riser of the invention may either be a “production riser” for crude oil or gas, or else it may be a water injection riser providing a connection with an underwater pipe resting on the sea bottom, or indeed it may be a “drilling” riser for providing a connection between the floating support and a wellhead located on the sea bottom.
In FPSOs, where multiple lines are generally installed, it is necessary to implement bottom-to-surface connections either of the hybrid tower type, or else of the “catenary” type.
When the bottom-to-surface connection pipe provides a direct connection between a floating support and a contact point on the sea bottom, which point is offset from the axis of said support, the pipe takes up a so-called “catenary” configuration under its own weight, thereby forming a curve whose radius of curvature decreases going down from the surface to the point of contact with the sea bottom, and the axis of said pipe forms an angle α with the vertical whose value lies generally in the range 10° to 20° at the floating support, and varying to reach an angle that is theoretically 90° at the sea bottom, corresponding to an ideal position which is substantially tangential to the horizontal, as explained below.
Catenary type connections are generally made using flexible pipes, but they are extremely expensive because of the complex structure of the pipe.
Thus, it has been the practice to develop substantially vertical risers so as to ensure that the flexible connection in a catenary configuration leading to the floating support is kept close to the surface, thus making it possible to minimize the length of said flexible pipe, and also to minimize the forces which are applied thereto, thereby considerably reducing its cost.
In addition, when producing oil, said crude oil travels over long distances, e.g. several kilometers (km), and it is desirable to provide a very high degree of insulation firstly to minimize any increase in viscosity which would lead to a reduction in the hourly production rate of the well, and secondly to avoid the flow becoming blocked by paraffin being deposited or by hydrates forming when the temperature of the oil drops to around 30° C. to 40° C. These phenomena are all the more critical, particularly off West Africa where the crude oils are of the paraffinic type, given that the temperature of water at the bottom of the sea is about 4° C.
Once the depth of water reaches or exceeds 800 meters (m) to 1000 m, it becomes possible to make said bottom-to-surface connection by means of a thick-walled rigid pipe since the considerable length of the pipe gives it sufficient flexibility to obtain a satisfactory catenary configuration while remaining within acceptable stress limits.
Such rigid risers made of thick strong materials, and disposed in catenary configurations, are commonly referred to as steel catenary risers, and in the present device, the abbreviation “SCR” or the term “catenary riser” are used regardless of whether the material is steel or some other material such as a composite material.
Such SCRs or catenary risers are much simpler to make than are flexible pipes, and they are therefore less expensive.
The geometrical curve formed by a pipe of uniform weight in suspension and subjected to gravity is known as a “catenary” and is a mathematical function of the hyperbolic cosine type (Cos hx=(ex+e−x)/2) relating the abscissa and the ordinate of an arbitrary point along the curve using the following formulae:y=R0(cos h(x/R0)−1)R=R0(Y/R0+1)2
in which:    x represents the distance in the horizontal direction between said contact point and a point M on the curve;    y represents the height of the point M (x and y are thus the abscissa and the ordinate of a point M on the curve relative to an orthogonal frame of reference whose origin is at said point of contact);    R0 represents the radius of curvature at said point of contact, i.e. at the point where the tangent is horizontal; and            R represents the radius of curvature at the point M (x,y).        
Thus, curvature varies along the catenary from the surface where its radius has a maximum value Rmax down to the point of contact where its radius has a minimum value Rmin (or R0 in the above formula). Under the effect of waves, wind, and current, the surface support moves sideways and vertically, which has the effect of raising and lowering the catenary-shaped pipe at the sea bottom.
Thus, the pipe presents a radius of curvature which is at a maximum at the top of the catenary, i.e. at the point where it is suspended from the FPSO, which radius of curvature is generally at least 1500 m, and in particular lies in the range 1500 m to 5000 m, and it decreases going down to its point of contact on the sea bed. At this location, the radius of curvature is at a minimum in the portion of the pipe that is in suspension. However, in the adjacent portion of the pipe that is lying on the sea bottom, said pipe being ideally in a straight line, the radius of curvature is theoretically infinite. In fact, said radius is not infinite but is extremely large, since some residual curvature remains.
Thus, as the floating support moves on the surface, the point of contact moves forwards and backwards and, in the zone that is lifted off and placed back on the sea bottom, the radius of curvature passes successively from a minimum value Rmin to a value that is extremely high, and tending to infinity in an ideal configuration where the undersea pipe rests on the sea bottom substantially in a straight line.
This alternating flexing leads to fatigue phenomena that are concentrated throughout the zone constituting the bottom of the catenary, and the lifetime of such pipes is greatly reduced, and is generally incompatible with the lifetimes desired for bottom-to-surface connections, i.e. 20 years to 25 years, or even more.
In addition, it is found that during these alternating movements of the point of contact, the stiffness of the pipe which is associated with the above-mentioned residual curvature will, over time, cause a furrow to be dug over the entire length that is raised and lowered, thereby creating a transition zone in which there exists a point of inflection where the radius of curvature which is at a minimum at the bottom of the catenary then changes sign in said transition zone and increases so as finally to reach an infinite value in the portion of the undersea pipe that is lying in a straight line on the sea bottom.
Such repeated movements over long periods give rise to a furrow of increasing depth in poorly consolidated sea beds of the kind commonly to be found at great depths, thereby modifying the curvature of the catenary and leading, if the phenomenon becomes amplified, to risks of damage to pipes, either where the undersea pipes rest on the sea bottom or else in the SCRs providing the connection between the surface and said undersea pipes resting on the sea bottom.
The most critical portion of the catenary is thus situated in the portion close to the point of contact, and the major fraction of forces in said low portion of the catenary are, in fact, generated by the movements of the floating support and by the excitations which occur in the upper portion of the catenary which is subject to current and to swell, with these excitations then all propagating mechanically along the pipe to the bottom of the catenary.
The currents that occur at the sea bottom, and the influence of swell at that depth, are known to be small and they do not give rise to significant hydrodynamic forces on the low portion of the catenary.
The support floating on the surface possesses considerable buoyancy and remains relatively insensitive to vertical loading generated by catenaries suspended from its side, but in contrast the horizontal tension H created by each of the catenaries must be counterbalanced, either by a balanced distribution of catenaries on the starboard and port sides, or by reinforcing the anchoring of the floating support on its side opposite from the catenaries.
Patent EP 0 952 301 describes an FDPU associated with a catenary bottom-to-surface connection through which a drill string passes, said bottom-to-surface connection serving not only as a guide but also as a return path for drilling mud carrying drilling debris. In the lower portion of said bottom-to-surface connection, where curvature is more accentuated, the rotating drill string rubs against the wall of the bottom-to-surface connection and runs the risk of damaging it, or even destroying it.
Thus, the problem posed is that of providing an undersea bottom-to-surface connection pipe capable of withstanding the fatigue that accumulates at its point of contact with the sea bottom, as created by movements of the support on the surface, and by the effects of swell and current, mainly-in the zone close to the surface where the effects of said swell and said current are generally largest.
This problem is made worse when the structure of the pipe includes a high performance insulation system, which system makes the pipe even more sensitive to problems of fatigue because of the complexity of its internal structure.
Thus, another problem posed is to provide a bottom-to-surface connection pipe in which the horizontal tension at the point of contact with the sea bed and at the level of said floating support is as small as possible, thereby minimizing the unbalance created on the anchoring of the floating support and the phenomena that lead to furrows being created at said point of contact.
Another problem posed is that of providing a bottom-to-surface connection pipe of the drilling riser type presenting improved mechanical characteristics, in particular for the purpose of reducing the risk of damage being caused to the riser by rotating drill strings inserted into the riser and rubbing against the inside wall thereof.
A solution to the problems posed is an undersea pipe of the riser type providing a connection between a floating support and the sea bottom, said riser being constituted by a rigid pipe of the catenary type extending from said floating support to a point of contact with the sea bottom, the pipe being characterized in that said catenary riser comprises a lower pipe portion terminating at said point of contact, the apparent weight per unit length in water of said lower pipe portion being less than that of the remainder of said pipe constituting said catenary riser.
The catenary pipe of the invention thus comprises at least two pipe portions corresponding to two different catenary curves, and it is thus referred to as a multi-catenary pipe.
More particularly, said lightweight lower portion of the pipe extends over a length of at least 100 m from said point of contact.
The lightening of said lower portion of the pipe compared with the remainder of the pipe has the effect of significantly increasing the radius of curvature R0 in the lower portion in the vicinity of the point of contact with the sea bottom, compared with the radius of curvature that would obtain if the lower portion of the pipe presented the same characteristics as said ordinary portion of the pipe. This increase in the minimum radius of curvature at the point of contact has the effect of considerably reducing both fatigue phenomena and also furrow-digging phenomena. During the forward and backward movements of the catenary, the portion of pipe is flexed in alternation to reach its minimum radius of curvature and then be returned substantially to a straight line, which gives rise to alternating stresses that are much smaller in the device of the invention than in the prior art because the minimum radius of curvature is larger, thereby reducing fatigue throughout the lifetime of the pipe, which generally exceeds 25 years. In addition, the furrow created at the bottom of the catenary by the residual curvature is less marked, thereby improving the long-term behavior of the bottom-to-surface connection.
For depths greater than 1000 m, said lightweight lower pipe portion preferably extends over a length lying in the range 200 m to 600 m.
More particularly, said lower pipe portion is lightened so as to have apparent weight per unit length in water that is at least 25%, and preferably 25% to 80% lighter than that of the remaining portion of the pipe which is adjacent thereto.